This invention relates to an automatic door closing device for use in a sliding type door which is supported by and slidable between a pair of vertical frames, and more particularly to a type thereof wherein a biasing means such as a spiral spring keeps the door in closing direction thereof and a buffer mechanism comprising a cylinder and a piston is provided to moderate closing speed of the door.
Automatic door closing means have alreadly been proposed in which spring energy accumulated in a spring member is released to close the sliding door under the control of a buffer mechanism using an air cylinder. However, such prior door closing device have their door closing performance reduced when the sliding door runs at a low speed due to the frictional resistance of the spring member. Second, such door closing devices use an air cylinder having a sliding part, for example, between the cylinder and the piston, to which foreign materials such as dust readily affix, and around which lubricating oil is readily wasted up. This results in an increase in frictional resistance to make the door closing performance hard and finally impossible. There are another problem has been raised. In case the force for opening the door is set small, the door may not be completely closed, due to the shortage of spring force, while if the closing force of the door set large, it may take much force to open the door.